A device to device (D2D) communications technology is a technology for direct communication between terminals. A difference between the D2D communications technology and a cellular communications technology lies in that, data sent by a terminal during D2D communication does not need to be transmitted by using a base station but is directly transmitted by using an air interface between terminals, thereby reducing a delay in forwarding data by using a base station.
A terminal that performs D2D communication may be referred to as a D2D device, multiple D2D devices form one D2D communication group, and D2D devices in a same D2D communication group perform D2D communication between one another. The D2D device has two resource scheduling modes: a base station-based resource scheduling mode (a first mode) and an autonomous resource selection mode (a second mode). In the base station-based resource scheduling mode, a resource used by the D2D device for performing data transmission is scheduled by a base station in a cellular network. The D2D device needs to report, to the base station by using a buffer status report (BSR), a volume of D2D data to be sent, and the base station schedules a corresponding resource for the D2D device according to the volume of the data in the BSR. To report the BSR to the base station, the D2D device also needs to use a particular uplink resource, and the uplink resource is also allocated by the base station to the D2D device.
A feature of the D2D communication is that a D2D device may simultaneously communicate with multiple other D2D devices (or multiple D2D communication groups). Therefore, a BSR reported by the D2D device may include a data volume of communication between the D2D device and the multiple other D2D devices (or the multiple D2D communication groups), causing that an uplink resource needed for sending the D2D BSR is relatively large. In the base station-based resource scheduling mode, an uplink resource that is used for sending the BSR and is allocated by a base station to the D2D device is limited, which causes that the D2D device may not be capable of sending information about an entire BSR to the base station.